This application claims the priority to and the benefit of Korean Patent Application No. 10-2005-0034221, filed on Apr. 25, 2005, in the Korean Intellectual Property Office, the entire content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a cylindrical lithium secondary battery and a method of fabricating the same.
2. Description of the Related Art
Recently, compact and lightweight electronic devices such as cellular phones, laptop computers, and camcorders are being widely developed and produced. Such portable electronic devices are typically provided with a battery pack so that the devices can be operated anywhere. The battery pack has at least one battery to drive the portable electronic devices for a predetermined time period.
A rechargeable secondary battery is usually produced with consideration given to cost efficiency. For example, rechargeable secondary batteries such as a Ni—Cd battery, a Ni-MH battery, a Li battery, and a Li-ion battery are available in the art.
Lithium secondary batteries have a high energy density per a unit weight and an operation voltage of 3.6V, which is triple that of a Ni—Cd battery or a Ni-MH battery. Therefore, lithium secondary batteries are increasingly used in the art.
In a lithium secondary battery, lithium based oxide is used as a positive electrode activation material, while a carbon based material is used as a negative electrode activation material. Generally, lithium secondary batteries are classified as liquid electrolyte batteries or polymer electrolyte batteries depending on the electrolyte used therein. A battery using liquid electrolyte is called a lithium ion battery, and a battery using polymer electrolyte is called a lithium polymer battery. In addition, lithium secondary batteries are fabricated in a variety of shapes such as cylindrical, rectangular, pouch types, and the like.
Typically, cylindrical lithium secondary batteries include an electrode assembly formed by winding a positive electrode plate to which a positive electrode tap is attached, a negative electrode plate to which a negative electrode tap is attached, and a separator interposed between the positive and negative electrode plates to prevent a short circuit and allow only lithium ions to move. Additionally, a cylindrical case may be provided to store the electrode assembly and electrolyte may be injected into the cylindrical case to allow lithium ions to move.
A cylindrical lithium secondary battery may be formed as follows. First, the positive electrode plate to which the positive electrode tap is attached, the negative electrode to which the negative electrode tap is attached, and a separator are stacked and wound to provide a cylindrical electrode assembly. In this case, the positive electrode tap is disposed at a winding start position, and the positive and negative electrode taps are in the shape of a plate.
Subsequently, the electrode assembly is inserted into a cylindrical case for a lithium secondary battery so that the electrode assembly does not separate. Then, electrolyte is injected into the cylindrical case and the case is sealed completing the cylindrical lithium secondary battery.
However, in a conventional cylindrical lithium secondary battery, since the positive electrode tap is disposed at a winding start position, the shape of the electrode assembly deviates from a cylindrical shape as it is wound. More specifically, the electrode assembly may tend to resemble an ellipsoid instead of a cylinder, preventing or inhibiting insertion of the electrode assembly into the cylindrical case.
Also, a protrusion formed on the electrode assembly due to the geometrical deviation from a cylindrical shape may result in scratches on the cylindrical case or on the electrode assembly when inserting the electrode assembly into the cylindrical case, leading to potential battery defects.